1. Field of the Invention
The present invention relates to the formation of oxide coatings on the surfaces of materials, particularly the formation of coatings with controlled hydrophilic/hydrophobic surface characteristics, especially by forming hydrophilic oxide coatings of inorganic, semimetallic, and/or metallic oxides, and most particularly the formation of these controlled, e.g., hydrophilic oxide coatings on particles to adjust the surface and/or chemical properties of the particles. The particles may include such materials as fillers, pigments, lakes, catalysts, matting agents, optical diffusive agents, strengtheners, magnetic particles, catalysts, reflective materials, film or sheet surfaces, fibers, filaments, and many other forms of materials and especially particulate additives.
2. Background of the Art
Particles are conventionally and conveniently added to many different types of compositions and products where the particles are intended to remain in particulate form after the manufacturing or finishing of the product. Product uses of particulates cover such diverse technical utilities as fillers, colorants (e.g., pigments and lakes), matting agents, antislip agents, optical diffusing agents, strengthening agents, viscosity modifiers, reflective particles, magnetic particles, carriers for other compounds and materials, and other types of composition additives. There are a number of problems which have long been associated with the use of particles in other formulations. The composition of the particle may react with ingredients in the formulation and detract from a desired activity of a component, reaction of an ingredient of the formulation with the particle may adversely affect the properties (physical or chemical) of the particle, the particle may be incompatible with the formulation and not form stable dispersions or suspensions, or the particles may agglomerate within the formulation and not remain in the appropriate size for their utility. These problems have been addressed over the years by many different techniques and chemical efforts.
Amongst the control techniques which have been used to overcome these problems are the application of coatings to the surface of the particles, using coupling agents on the surface of the particles, physically modifying the surface of the particles, chemically modifying the innate composition on the surface of the particles, and/or modifying the formulation to accommodate the particle. The last is one of the least desirable methods of controlling the behavior of the particles in the formulation as it requires alteration of the fundamental composition or at least tinkering of the composition which can alter its essential properties. Similarly coatings on the surface of the particles or reaction of materials (such as coupling agents) onto the surface may affect more than the mere hydrophilic/hydrophobic balance of the properties of the surface and may even add undesirable functionality into the particle surface which can directly affect the performance of the composition. Physical modification of the surface of the particle can easily affect its optical properties, and coatings on the particles can be difficult and expensive to achieve, with results that can vary greatly from one type of particle to another.
Zinc oxide is a particular pigment which has received significant attention with respect to its commercial use with coatings thereon. As described in U.S. Pat. No. 5,486,631, zinc oxide has been coated with organic oils, such as the organic oil tri-(octyldodecyl)citrate, to provide a stable dispersion of the zinc oxide which can be used in sun screens. The organic oil coating is not permanent and therefor not durable because the oil neither reacts with itself to form a coating nor reacts with the zinc oxide itself. The oil also disrupts the uniformity of the zinc oxide particles themselves as they are distributed on the skin in the sun screen formulations. This U.S. Pat. No. (5,486,631) describes the use of a specific type of reactive silicone to hydrophobize the surface of zinc oxide particles. Silicone compounds are shown of the formula: ##STR1## wherein Me is methyl, R is an alkyl group having one to ten carbon atoms,
R is methyl or ethyl, and PA0 a is an integer within the range of to 12.
It is at least noteworthy that these types of compounds used in the prior art, as represented by this reference (U.S. Pat. No. 5,486,631), hydrophobize surfaces while the practice of the present invention is able to increase the hydrophilicity of the surfaces and even render them hydrophilic using the different process of the present invention.
The specific silicone compounds are mixed with the zinc oxide particles then the mixture is heated at a temperature of between 40 and 100.degree. C. for two to ten hours. The silicone compound is indicated as forming direct bonds between the silicon atom and zinc oxide crystals, of course, with additional valences in the silicon atom satisfied by original groups from the specific silicone compounds.
U.S. Pat. No. 5,536,492 describes the use of hydrophobized zinc oxide particles such as those made in U.S. Pat. No. 5,486,631 in sun screen products applied to the skin.
U.S. Pat. Nos. 5,458,681 and 5,368,639 describe organosilicon-treated pigments, a process for making such treated pigments, and cosmetic compositions made with those treated pigments. Linear reactive alkylpolysiloxane having particular substituent groups (for example, including amino, imino, halogen, hydroxyl and/or alkoxy groups) are oriented and adsorbed to the surface of a pigment or extender by heat treatment. The alkylpolysiloxane is identified as having a particular range of polymerization from 25 to 100 and a Mw/Mn ratio of 1.0 to 1.3. The silicone is described as firmly adhered to the surface of the pigment.
Coatings of organo-metallic compounds, organic non-metallic compounds, inorganic oxides, and inorganic non-oxide materials have been widely used in the formation of abrasion resistant and/or solvent resistant coatings on surfaces, including the surfaces of polymers. The range of materials which have been use for this purpose covers a wide spectrum of materials including, but not limited to, acrylates, polysiloxanes, poly(ambifunctional)silanes, esters of metals (e.g., esters of titanium, aluminum, zirconium, etc.) alone or in combination with other ingredients such as ambifunctional silanes (e.g., U.S. Pat. No. 4,084,021).
Silane and titanate compounds are also well known in the general literature as coupling agents or coupling moieties for chemically bonding materials to each other, including the bonding of specific functional moieties to particular surfaces. In this vein, Okano, Tamon; Tsukiyama, Katsunori; Konishi, Hisatoshi; and Kiji, Jitsuo, "Versatile Polymer-Bound Rhodium Catalysts. Facile Hydrogenation of Aromatic Compounds in the Liquid Phase", Chemistry Letters, 1982, pp. 603-606 show the attachment of phosphino groups to silica (glass) surfaces by reacting an alpha-phosphino-propyltrimethoxyalkane to the glass to form a tethered moiety having an alpha-phosphinopropyl group tethered to the glass through a trisiloxy bond.
Amphoteric/ambifunctional compounds of silanes and titanates, as well as other classes of bifunctional compounds, are also used as coupling agents, with the various end groups selected for appropriate reactions to assist in bonding or stabilizing two dissimilar surfaces or materials. Of particular interest to the practice of this invention is U.S. Pat. No. 4,756,906 which describes an adjustable cosmetic composition, and in its background disclosure describes the use of adsorption (including chemisorption) by Van der Waals forces, dipole-dipole attraction, or hydrogen bonding to associate additives to the cosmetic composition and coupling agents. Titanate and silane coupling agents are extensively disclosed. The list and formulae of materials within these classes on columns 5 and 6, as well as the classes of pigments described on columns 6 and 7 is hereby incorporated by reference for that disclosure.
U.S. Pat. No. 5,520,952 describes a method for producing protective film coatings, particularly on electronic parts and devices. The method comprises the application of a liquid coating composition onto a surface, the principal component of the coating composition comprising a partial cohydrolysis-condensation product of a tetraalkoxy silane and a functional alkoxy silane(e.g., tetraethoxy silane and 3-methacryloxypropyl trimethoxy silane, respectively). The coated article is heated to effect curing of the composition. Finely divided inorganic filler (e.g., a colloidal silica) may be added to the liquid coating composition to even further increase the hardness of the cured coating. U.S. Pat. No. 5,496,402, which is commonly assigned with U.S. Pat. No. 5,520,952 similarly shows stable coating compositions comprising two different hydrolyzable silanes, one a triester and the other a tetraester silane. A third commonly assigned patent, U.S. Pat. No. 4,865,649 also describes a coating solution for forming a silica-based coating film, the coating solution comprising an organic solution of a hydrolyzate of an alkoxy silane mixture of at least two different kinds of d-, tri, and tetra-alkoxy silane compounds in a specified molar ratio. The coating compositions are cured in the presence of water without the need for an acid catalyst to effect cohydrolysis of the silanes. Another related patent, U.S. Pat. No. 4,277,525, describes liquid coating compositions for the formation of silica coating films, the coating composition comprising an alkoxy-containing silane, a lower carboxylic acid and an alcohol in the presence of a reaction accelerator (which is an acid different from the carboxylic acid).
U.S. Pat. No. 5,344,751 describes an antistatic protective coating comprising a mixture of sodium metasilicate, a silica sol, and a silane coupling agent. Similar compositions comprising orthosilicates are known in the patent literature also.
UV curable coatings comprising a photoinitiator, hydrolysis product of silylacrylate and aqueous colloidal silica is shown (also in combination with a polyfunctional acrylate) to provide adherent and abrasion resistant coatings, particularly for polycarbonate resins in U.S. Pat. No. 4,455,205.
Abrasion resistant coatings and coatings of metal oxides have also been deposited on surfaces by vapor deposition techniques in which metal and oxygen (and/or other reactive materials such as sulfur) are evaporated and reacted/deposited onto surfaces to form coatings. Apparatus and processes for this type of coating are shown, for example, in U.S. Pat. Nos. 4,430,366 and 4,405,678. These coatings may even have gradations in their relative atomic distribution of materials within the vapor-deposited layers.
It is known in the art that pigments may be desirably provided with coatings to protect them from formulations into which they are compounded, as previously noted. U.S. Pat. No. 5,482,547 describes the application of a tenacious coating of an alkyl silicate onto pigments, the coating being fixed onto the surface of the pigment by adsorption of an alkyl silicate on a layer that coats the surface of the pigment particles and consists essentially of partially hydrolyzed organic compounds of a specific formula comprising group 4A or 4B elements and chelates of a specific formula, the chelates being titanium or tin chelates. The alkyl silicate may be hydrolyzed before, during or after adsorption. Good gloss and rheologic properties are asserted to be a result of this particular coating.
Each of these processes and other commercial processes have their respective advantages and disadvantages. One surprising aspect of this field of technology is that even when similar starting reagents are used for the different processes, different products and/or different properties may be produced in the final article because of variations in conditions. Alternative coatings, coating solutions and coating processes which may provide a wider and different range of properties are still desirable.